1. Field of the Invention
This invention relates to the field of detection of particles in liquid, and particularly to the detection of particles in ultra-pure liquids. More particularly this invention relates to the detection of submicron particles in ultra-pure liquids.
2. Description of the Related Art
Many fields utilize liquids whose purity is critical. For example, in the nuclear power industry, water is used in a nuclear reactor in the nuclear power generation process. Purity of this water is of utmost criticality to ensure the safe and effective operation of the nuclear reactor. Liquids used in silicon chip manufacture are also preferably ultra-pure. Certain other liquid chemicals are manufactured to be ultra-pure for various other applications.
Generally, after distillation, filtration and treatment for organic and ionic removal, liquids required to be ultra-pure may be tested for purity. In practice, however, ultra-pure liquids are generally only tested for ionic content using well known ionic content detection methods. Particulate purity, on the other hand, is more difficult to accurately test. Many times the particulate purity of the liquid is simply assumed by relying upon the integrity of the filtration process.
Optical techniques such as those employing optical scattering and the use of optical microscopes have been used in an attempt to test the particulate purity of a liquid. Optical techniques, however, are ineffective if the liquid medium is colored or if the access to the liquid is not transparent. In many applications, the liquids are stored in metal containers or pipes, so testing using optical techniques entails removal of at least a portion of the liquid to a transparent vessel. In applications such as the nuclear power plant, removal of the reactor water generally requires a shut down of the nuclear reactor and thus the entire power plant. Shutting down to test for particulate purity in the reactor water on a regular basis is impractical and therefore, the optical technique is unworkable in this and many other environments.
Electron microscopy techniques have also been employed. Like optical techniques, however, this method requires the removal of a sample for placement in the viewing area of the microscope. Further, in electron microscopy, a vacuum environment is required. This technique may also be unworkable in many environments. Laser techniques have also been attempted but are generally ineffective in opaque media.
Attempts to use acoustic scattering have also been made. Acoustic scattering involves the detection of acoustic waves echoed off of bodies embedded in a three-dimensional, infinitely propagating media when presented with an acoustic signal. Differing shaped and sized bodies produce differing acoustic waves. The acoustic waves received, or echoed back, are referred to as an acoustic signature for the particular size and shape of the body in the media. Acoustic scattering as a method for testing the particulate purity of liquids, however, has generally been ineffective because the acoustic signature of liquid borne particles, especially submicron particles, is very weak and difficult to detect.